In the case of known optical communication systems, the transmitted optical signal has been converted to an electric signal, so as to enable the signal to be detected and amplified. The converted signal has been retransmitted, or a new signal has been transmitted, with the aid, e.g., of light emitting diodes or laser diodes. These known systems are relatively complicated and expensive, and consequently endeavours have been made to avoid the use of optoelectrical conversions and to increase the part played by optics in communication systems. These endeavours have resulted in difficulties, due in part to significant attenuation or damping of the optical signal, primarily in optical communication systems which included star switches, and consequently these have been provided with optical amplifiers. A communication system in which the amplification of an optical signal is effected solely in an optical fashion is described in an article "100" Mbit/s Laser diode Terminal with Optical Gain for Fibre-Optic Local Area Networks", written by A. Alping et al, and published in Electronics Letters, 13th September 1984, Vol. 20, No. 19, pages 794, 795. In an experiment, there was used a first laser diode for signal transmission and a second laser diode was used as a travelling wave amplifier for detecting the signal and retransmitting the same subsequent to amplification. Other optical systems are known in which a laser diode is used both to transmit and to amplify a signal. For example, UK Patent Application No. GB-A-2 183 417 describes a distance meter which includes a laser diode capable of transmitting a light pulse through one end surface of the diode and onto a reflective target. The other end surface of the laser diode is connected to an input of a directional coupler switch, which can be alternated between its terminal or end positions, i.e. its cross state and bar state. One output of the coupler switch is connected to a photodetector and the other output is provided with a mirror. In one terminal position of the switch, a cavity is formed between the switch mirror and one reflective end surface of the laser diode. The laser diode, which is connected to a drive source, transmits the light pulse, which is reflected by the target back to the laser diode. The directional coupler switch is switched to its other terminal state during this transmission/reflection delay time. The reflected light pulse passes the laser diode, which now functions as an amplifier. The' light pulse amplified by the laser diode is coupled to the photodetector by the switch, so that the light pulse can be detected and its delay time measured. One drawback with both of the afore-described arrangements is that they are relatively complicated and require the provision of more optical amplifiers or detectors. The distance meter taught by the aforesaid UK patent application is not suited for fibre optical communication systems, since the end surfaces of the laser amplifier reflect an incoming light signal and thereby generate undesirable disturbance signals.